One field of application for the invention is, for instance, the treatment of scoliosis. However, the invention may also be useful to treat abnormal curvatures, deficiencies or other abnormalities of the spine, including instability of spinal segments or degenerated intervertebral discs.
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the sacrum (situated beneath the lumbar vertebrae) to the cervical vertebrae. Each vertebra has an anterior part, which is the vertebral body, and a posterior part, which is the vertebral arch (or neural arch), these parts enclosing the vertebral foramen.
The vertebral arch is formed by a pair of pedicles, a pair of laminae, and support articular, transverse and spinous processes. These processes project opposite to the vertebral foramen. A spinous process is also called a neural spine.
When the vertebrae are articulated with each other, the vertebral bodies form a strong pillar for the support of the head and trunk, and the vertebral foramen constitutes a canal for protecting the spinal cord (or medulla spinalis). In between every pair of vertebrae, there are two apertures called intervertebral foramina, one on either side of the vertebral axis, for passing the spinal nerves and vessels.
When the spine of a person has abnormal curvature or other deficiencies, the vertebrae are typically too close together or spaced too far apart, and there is a need to stabilize the vertebrae in the correct position relative to one another. Mainly, there is either a need to compress the vertebrae (i.e. to bring them and hold them closer together) or a need to distract the vertebrae (i.e. to space them and keep them further apart). To do this, various kinds of devices known in the art may be used.
Typically, such known devices include at least two anchors configured to be fastened, respectively, to two vertebrae, and a rod for connecting the anchors together, thereby providing stabilization between the vertebrae.
In a first kind of device, known in the art, said anchors are hooks that rest on the vertebral laminas and go along the internal wall of the vertebral foramen. Examples of such devices are disclosed, for instance, in PCT application no 2005/023126 or U.S. Pat. No. 4,269,178.
A hook has the advantage of providing a rigid and strong anchoring to the vertebra, but the physician (or other operative) may have difficulty in placing the hook correctly on the vertebra. Moreover, there is a risk that the hook may disengage from the vertebra. Such a risk sometimes leads the physician to use more hooks than necessary.
In another known example of device, disclosed in PCT application no 2009/144663, at least one of said anchors is an anchoring assembly including a body, a hook and a flexible ligament. The hook extends from the bottom face of the body. A hole is provided through the body, and extends from one lateral face of the body to the opposite lateral face. In use, the physician positions the hook on a first bony portion of a vertebra, and engages the flexible ligament through the hole so as to form a loop around a second bony portion, which is different from the first bony portion, of the same vertebra or another vertebra. A first drawback is that, the physician needs to find a convenient place for the hook and another convenient place for the flexible ligament. Moreover, since the physician applies tension to the flexible ligament after positioning the hook on the first bony portion, the flexible ligament tends to pull the body and the hook towards the second bony portion. In situations where the main direction of the tension is not aligned with the hook, this may lead to an undesired movement of the hook with respect to the first bony portion and, sooner or later, the assembly may become loosened.
In another kind of device, known in the art, said anchors are such as those disclosed in PCT application no 2009/047352. That kind of anchor comprises a blocking body and an elongate flexible member. In use, the flexible member is passed around a vertebra and through the blocking body and a rod is loaded into the blocking body. The ends of the flexible member are pulled so as to apply tension to the flexible member, and the flexible member and the rod are simultaneously fastened to the blocking body by means of the same fastening system, portion(s) of the flexible member being clamped between the rod and the blocking body. In such a system, the flexible member needs to be continuously kept under tension until the rod is fastened to the blocking body. Moreover, the flexible member may be anchored less rigidly and strongly than a hook.